a) Field of the Invention
The present invention relates to a pattern forming technique which uses an amorphous carbon (a-C), and more particularly to a pattern forming technique which includes the process of forming and/or etching an a-C film. In this specification, the term "amorphous carbon (a-C)" is intended to include those having dangling bonds terminated with hydrogen or the like.
b) Description of the Related Art
To improve the integration degree of semiconductor large scale integrated (LSI) circuits, it is necessary to miniaturize circuit elements, wirings, and the like. To obtain fine structures, it is necessary to narrow a minimum line pattern width and improve a patterning precision. It has been desired therefore to improve a resolution of photolithography.
The resolution of photolithography depends on a numerical aperture and a wavelength of light. As the numerical aperture is increased, the depth of focus becomes shallow. Photolithography for a substrate having steps such as multilayered wirings requires some depth of focus, so that an increase of the numerical aperture is limited. It is therefore preferable to shorten the wavelength of light in order to improve the resolution.
In photolithography of a semiconductor manufacturing technique, the wavelength of exposure light has shortened from the g line to i line (365 nm) of mercury, to KrF excimer laser beam (248 nm), and to ArF excimer laser beam (193 nm).
Photolithography is used in some cases for a layer whose underlying layer is made of Si, refractory metal silicide such as tungsten silicide, or refractory metal such as tungsten. These materials have a high reflectance in the ultraviolet "(UV)" wavelength range. A reflectance of Si increases greatly from the visual light wavelength range to the ultraviolet wavelength range. In some cases, such a high reflectance material is formed under a transparent layer made of silicon oxide or the like.
If an underlying layer of a photoresist layer has a high reflectance, strong light reflected from the underlying layer lowers a photolithography precision. The precision is lowered more when the surface of an underlying layer has a slanted area. It has been desired to form an anti-reflection (reflection reducing) film which efficiently reduces reflected light from an underlying layer. A photolithography technique using an amorphous carbon film as an antireflection film has been proposed by Y. Suda et al, Proc. SPIE, 1674, pp. 350-361 (1942). This paper describes a technique of forming an amorphous carbon film as an antireflection film on a high reflectance film or on a transparent film formed on a high reflectance film, and values of real parts (refractive index n) and imaginary parts (extinction coefficient k) of complex refractive index (N) suitable for an antireflection film.
FIGS. 9A to 9E illustrate an example of a conventional photolithography technique using an a-C film as an antireflection film. As shown in FIG. 9A, a polycide layer of a polycrystalline silicon film 102 and tungsten silicide film 102 is formed on an underlying layer 101. This polycide layer has a high reflectance for an ultraviolet ray.
On the tungsten silicide film 103, an a-C film 104 is deposited to a predetermined thickness to form an antireflection film. A photoresist layer 105 of negative type is coated on the a-C film 104.
Ultraviolet rays 106 are applied to a selected area of the photoresist layer 105. Photoresist exposed to ultraviolet rays undergoes chemical reactions and becomes insoluble to developing liquid.
Reflection of light passed through the photoresist layer 105 is reduced because the surface of the tungsten silicide layer 103 is covered by the a-C film 104. Since the amount of reflected light is reduced, a resolution can be prevented from being lowered by reflected light as compared to the case where a photoresist layer 105 is formed directly on a tungsten silicide layer 103.
As shown in FIG. 9B, the photoresist layer 105 is developed by developing liquid to form a resist pattern 105a, by leaving only the area of the light-exposed photoresist layer. The developing liquid is, for example, alkali solution which does not etch the a-C film 104.
As shown in FIG. 9C, the exposed area of the a-C film 104 is etched by reactive ion etching (RIE) with O.sub.2 gas to form an a-C pattern 104a having the same pattern as the resist pattern 105a. Carbon reacts with O and is vaporized in the form of CO.sub.2. The tungsten silicide layer 103 is not etched by O.sub.2 RIE.
As shown in FIG. 9D, by using the resist pattern 105a and a-C pattern 104a as an etching mask, the tungsten silicide layer 103 and polycrystalline silicon layer 102 are selectively etched to form a tungsten silicide pattern 103a and a polycrystalline silicon pattern 102a respectively having a desired shape.
After the tungsten silicide layer and polycrystalline silicon layer are patterned into a desired shape, both the resist pattern 105a and a-C pattern 104a are removed at the same time by ashing with oxygen plasma as shown in FIG. 9E.
As described above, an a-C film formed on a high reflectance layer functions as an antireflection film, enabling to reduce reflected light and prevent a resolution from being lowered.
However, additional processes are required for forming and removing an a-C film as an antireflection film. In addition, there are many uncertain points regarding how an a-C film having physical constants suitable for an antireflection film can be formed.